I, like many customers here, have had a major increase in data usage with the iPhone 5. In fact I blew through 1.3 gigs of data in just a couple weeks even though I use wifi 80% of the time.
A couple days ago I restored my phone to its factory settings through iTunes, which installed a fresh version of iOS 6.0.1. This seems to have cured the problem with data and wifi. When I got the iPhone 5 I had restored it from a backup which was from my iPhone 4. Somehow installing a fresh version of iOS 6 on my phone has fixed the problem.
I’m guessing there is a glitch when installing an iPhone 4 backup onto a iPhone 5.